Method for device-to-device (d2d) operation performed by terminal in wireless communication system and terminal using the method

ABSTRACT

A method for device-to-device (D2D) operation performed by a terminal in a wireless communication system, and a terminal using the method are provided. The method comprises: receiving, from a cell, resource pool information indicating a first resource pool which may be used in transmitting a D2D signal within the coverage of the cell; and transmitting a D2D signal by using overlapping resources between the first resource pool, and a second resource pool which may be used in the D2D operation outside the coverage of the cell.

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to wireless communication, and moreparticularly, to a method for a D2D operation performed by a terminal ina wireless communication system and a terminal using the method.

Related Art

In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3^(rd) Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D22) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D operation.

D2D operation may have various advantages in that it is communicationbetween devices in proximity. For example, D2D UE has a high transferrate and a low delay and may perform data communication. Furthermore, inD2D operation, traffic concentrated on a base station can bedistributed. If D2D UE plays the role of a relay, it may also play therole of extending coverage of a base station.

Meanwhile, the terminal is configured with a first resource which can beused for the D2D operation in network coverage from a network. Further,the terminal is configured with a second resource which can be used forthe D2D operation outside the network coverage. A part where the firstand second resources overlap with each other may be present or notpresent. When the terminal enters the inside from the outside of thenetwork coverage, a method for guaranteeing reliability of the D2Doperation while considering various relationships between the firstresource and the second resource and a terminal using the method arerequired.

SUMMARY OF THE INVENTION

The present invention provides a method for a device-to-device (D2D)operation performed by a terminal in a wireless communication system,and the terminal using the method.

In one aspect, provided is a method for a device-to-device (D2D)operation performed by a terminal in a wireless communication system.The method includes receiving resource pool information indicating afirst resource pool usable for transmitting a D2D signal within coverageof a cell from the cell and transmitting the D2D signal by using aresource which overlaps between a second resource pool usable for theD2D operation outside the coverage of the cell and the first resourcepool.

The terminal may select a specific resource within the resource whichoverlaps between the first resource pool and the second resource pool totransmit the D2D signal.

The terminal may transmit the D2D signal to another terminal outside thecoverage of the cell by using the selected specific resource.

The D2D signal may be a signal for D2D communication or a signal for D2Ddiscovery.

When the resource is not present, which overlaps between the firstresource pool and the second resource pool, the terminal may transferthe resource pool information indicating the first resource pool toanother terminal outside the cell coverage.

Scheme information indicating any one scheme of a first scheme and asecond scheme may be received from the cell, and whether the resourcepool information is transferred to another terminal outside the cellcoverage may be determined according to the scheme indicated by thescheme information.

The first scheme may be a scheme in which the terminal transmits the D2Dsignal by using the resource which overlaps between the first resourcepool and the second resource pool without transferring the resource poolinformation to another terminal outside the cell coverage, and thesecond scheme may be a scheme in which the terminal transmits the D2Dsignal by using the first resource pool by transferring the resourcepool information to another terminal outside the cell coverage.

The scheme information may be included in system information.

A threshold value for the overlapped resource may be received from thecell.

When the quantity of resources which overlap between the first resourcepool and the second resource pool is smaller than the threshold value,the resource pool information indicating the first resource pool may betransferred to another terminal outside the cell coverage.

The threshold value for the cell coverage may be received from the cell.

The threshold value for the cell coverage may include a first thresholdvalue used for determining an inside boundary of the cell coverage and asecond threshold value used for determining an outside boundary of thecell coverage.

Only when the terminal is positioned between the inside boundary and theoutside boundary, the resource pool information may be transferred toanother terminal outside the cell coverage.

The terminal may periodically transfer the resource pool information.

In another aspect, provided is a terminal. The terminal includes a radiofrequency (RF) unit transmitting and receiving a radio signal and aprocessor operated in association with the RF unit. The processorreceives resource pool information indicating a first resource poolusable for transmitting a D2D signal within coverage of a cell from thecell, and transmits a D2D signal by using a resource which overlapsbetween a second resource pool usable for a D2D operation outside thecoverage of the cell and the first resource pool.

According to the present invention, a resource to be used for a D2Doperation is determined by considering a relationship between a firstresource which can be used for the D2D operation inside network coverageand a second resource which can be used for the D2D operation outsidethe network coverage. Since the D2D operation is performed by applying aresource depending on an appropriate method according to the firstresource and the second resource, reliability of the D2D operationincreases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

FIG. 9 shows a basic structure for ProSe.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

FIG. 13 is an embodiment of a ProSe discovery process.

FIG. 14 is another embodiment of a ProSe discovery process.

FIG. 15 illustrates the UE-NW repeater.

FIG. 16 illustrates the UE-UE repeater.

FIG. 17 illustrates the number of hops of the UE-NW repeater.

FIG. 18 illustrates the number of hops of another UE-NW repeater.

FIG. 19 illustrates a case in which the present invention can beapplied.

FIG. 20 illustrates a D2D operating method of a terminal according to anexemplary embodiment of the present invention.

FIG. 21 illustrates an example in which the method of FIG. 20, that is,the method 1 is applied.

FIG. 22 illustrates a D2D operating method of a terminal according toanother embodiment of the present invention.

FIG. 23 illustrates an example in which the method 2 is applied.

FIG. 24 illustrates a terminal that transfers the resource poolinformation indicating the first resource pool to the terminals outsidethe cell coverage.

FIG. 25 illustrates a D2D operation of a terminal according to yetanother embodiment of the present invention.

FIG. 26 is a block diagram illustrating a terminal in which anembodiment of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockTypel” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is in the RRC_IDLE state: the UE needs to have the valid        version of the MIB and the SIB1 in addition to the SIB2 to SIB8.        This may comply with the support of a considered RAT.    -   If UE is in the RRC connection state: the UE needs to have the        valid version of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel.Thereafter, if the UE has only to search for a suitable cell thatsatisfies a cell selection criterion, the UE selects the correspondingcell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

The cell selection criterion may be defined as below equation 1.

Srxlev>0 AND Squal>0

where:

Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−Pcompensation

Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))   [Equation 1]

Here, the variables in the equation 1 may be defined as below table 1.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Signalled values, i.e., Q_(rxlevminoffset) and Q_(qualminoffset), may beapplied to a case where cell selection is evaluated as a result ofperiodic search for a higher priority PLMN during a UE camps on a normalcell in a VPLMN. During the periodic search for the higher priority PLMNas described above, the UE may perform the cell selection evaluation byusing parameter values stored in other cells of the higher priorityPLMN.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.

Rs=Qmeas,s+Qhyst, Rn=Qmeas,s−Qoffset   [Equation 2]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

Referring to FIG. 8, UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

Hereinafter, a D2D operation will be described. In 3GPP LTE-A, a serviceassociated with the D2D operation is referred to as proximity basedservices (ProSe). Hereinafter, the ProSe is an equivalent concept to theD2D operation and the ProSe may be mixedly used with the D2D operation.Hereinafter, the ProSe will be described.

The ProSe includes ProSe direction communication and ProSe directdiscovery. The ProSe direct communication is communication performedbetween two or more proximate UEs. The UEs may perform communication byusing a protocol of a user plane. A ProSe-enabled UE implies a UEsupporting a procedure related to a requirement of the ProSe. Unlessotherwise specified, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE is a UEsupporting both of a function specified for a public safety and a ProSeprocedure, and the non-public safety UE is a UE supporting the ProSeprocedure and not supporting the function specified for the publicsafety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

Hereinafter, for convenience, the ProSe direct communication may bereferred to as D2D communication, and the ProSe direct discovery may bereferred to as D2D discovery.

FIG. 9 shows a basic structure for ProSe.

Referring to FIG. 9, the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

-   -   Interworking via a reference point toward the 3rd party        applications    -   Authorization and configuration of UE for discovery and direct        communication    -   Enable the functionality of EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of the ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of the EPC,        e.g., offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

-   -   PC1: a reference point between the ProSe application program        within the UE and the ProSe application program within the ProSe        APP server. This is used to define signaling requirements in an        application dimension.    -   PC2: a reference point between the ProSe APP server and the        ProSe function. This is used to define an interaction between        the ProSe APP server and the ProSe function. The update of        application data in the ProSe database of the ProSe function may        be an example of the interaction.    -   PC3: a reference point between the UE and the ProSe function.        This is used to define an interaction between the UE and the        ProSe function. A configuration for ProSe discovery and        communication may be an example of the interaction.    -   PC4: a reference point between the EPC and the ProSe function.        This is used to define an interaction between the EPC and the        ProSe function. The interaction may illustrate the time when a        path for 1:1 communication between types of UE is set up or the        time when ProSe service for real-time session management or        mobility management is authenticated.    -   PCS: a reference point used for using control/user plane for        discovery and communication, relay, and 1:1 communication        between types of UE.    -   PC6: a reference point for using a function, such as ProSe        discovery, between users belonging to different PLMNs.    -   SGi: this may be used to exchange application data and types of        application dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 10(b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 10(c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 10(d), UE A may be placed within coverage ofa first cell, and UE B may be placed within coverage of a second cell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 10.

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC_CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

As a method in which resources for announcement of discoveredinformation are allocated not specifically to a terminal, a base stationprovides a resource pool configuration for announcement of thediscovered information to terminals. The configuration is included in asystem information block (SIB) to be signaled by a broadcast scheme.Alternatively, the configuration may be provided while being included ina terminal specific RRC message. Alternatively, the configuration may bebroadcast signaling of another layer except for an RRC message orterminal specific signaling.

UE autonomously selects a resource from an indicated resource pool andannounces discovery information using the selected resource. The UE mayannounce the discovery information through a randomly selected resourceduring each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_CONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

FIG. 13 is an embodiment of a ProSe discovery process.

Referring to FIG. 13, it is assumed that UE A and UE B haveProSe-enabled application programs managed therein and have beenconfigured to have a ‘friend’ relation between them in the applicationprograms, that is, a relationship in which D2D communication may bepermitted between them. Hereinafter, the UE B may be represented as a‘friend’ of the UE A. The application program may be, for example, asocial networking program. ‘3GPP Layers’ correspond to the functions ofan application program for using ProSe discovery service, which havebeen defined by 3GPP.

Direct discovery between the types of UE A and B may experience thefollowing process.

1. First, the UE A performs regular application layer communication withthe APP server. The communication is based on an Application ProgramInterface (API).

2. The ProSe-enabled application program of the UE A receives a list ofapplication layer IDs having a ‘friend’ relation. In general, theapplication layer ID may have a network access ID form. For example, theapplication layer ID of the UE A may have a form, such as“adam@example.com.”

3. The UE A requests private expressions code for the user of the UE Aand private representation code for a friend of the user.

4. The 3GPP layers send a representation code request to the ProSeserver.

5. The ProSe server maps the application layer IDs, provided by anoperator or a third party APP server, to the private representationcode. For example, an application layer ID, such as adam@example.com,may be mapped to private representation code, such as“GTER543$#2FSJ67DFSF.” Such mapping may be performed based on parameters(e.g., a mapping algorithm, a key value and so on) received from the APPserver of a network.

6. The ProSe server sends the types of derived representation code tothe 3GPP layers. The 3GPP layers announce the successful reception ofthe types of representation code for the requested application layer IDto the ProSe-enabled application program. Furthermore, the 3GPP layersgenerate a mapping table between the application layer ID and the typesof representation code.

7. The ProSe-enabled application program requests the 3GPP layers tostart a discovery procedure. That is, the ProSe-enabled applicationprogram requests the 3GPP layers to start discovery when one of provided‘friends’ is placed in proximity to the UE A and direct communication ispossible. The 3GPP layers announces the private representation code(i.e., in the above example, “GTER543$#2FSJ67DFSF”, that is, the privaterepresentation code of adam@example.com) of the UE A. This ishereinafter called ‘announcement’. Mapping between the application layerID of the corresponding application program and the privaterepresentation code may be known to only ‘friends’ which have previouslyreceived such a mapping relation, and the ‘friends’ may perform suchmapping.

8. It is assumed that the UE B operates the same ProSe-enabledapplication program as the UE A and has executed the aforementioned 3 to6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery.

9. When the UE B receives the aforementioned ‘announce’ from the UE A,the UE B determines whether the private representation code included inthe ‘announce’ is known to the UE B and whether the privaterepresentation code is mapped to the application layer ID. As describedthe 8 step, since the UE B has also executed the 3 to 6 steps, it isaware of the private representation code, mapping between the privaterepresentation code and the application layer ID, and correspondingapplication program of the UE A. Accordingly, the UE B may discover theUE A from the ‘announce’ of the UE A. The 3GPP layers announce thatadam@example.com has been discovered to the ProSe-enabled applicationprogram within the UE B.

In FIG. 13, the discovery procedure has been described by taking intoconsideration all of the types of UE A and B, the ProSe server, the APPserver and so on. From the viewpoint of the operation between the typesof UE A and B, the UE A sends (this process may be called announcement)a signal called announcement, and the UE B receives the announce anddiscovers the UE A. That is, from the aspect that an operation thatbelongs to operations performed by types of UE and that is directlyrelated to another UE is only step, the discovery process of FIG. 13 mayalso be called a single step discovery procedure.

FIG. 14 is another embodiment of a ProSe discovery process.

In FIG. 14, types of UE 1 to 4 are assumed to types of UE included inspecific group communication system enablers (GCSE) group. It is assumedthat the UE 1 is a discoverer and the types of UE 2, 3, and 4 arediscoveree. UE 5 is UE not related to the discovery process.

The UE 1 and the UE 2-4 may perform a next operation in the discoveryprocess.

First, the UE 1 broadcasts a target discovery request message (may behereinafter abbreviated as a discovery request message or M1) in orderto discover whether specific UE included in the GCSE group is inproximity. The target discovery request message may include the uniqueapplication program group ID or layer-2 group ID of the specific GCSEgroup. Furthermore, the target discovery request message may include theunique ID, that is, application program private ID of the UE 1. Thetarget discovery request message may be received by the types of UE 2,3, 4, and 5.

The UE 5 sends no response message. In contrast, the types of UE 2, 3,and 4 included in the GCSE group send a target discovery responsemessage (may be hereinafter abbreviated as a discovery response messageor M2) as a response to the target discovery request message. The targetdiscovery response message may include the unique application programprivate ID of UE sending the message.

An operation between types of UE in the ProSe discovery processdescribed with reference to FIG. 14 is described below. The discoverer(the UE 1) sends a target discovery request message and receives atarget discovery response message, that is, a response to the targetdiscovery request message. Furthermore, when the discoveree (e.g., theUE 2) receives the target discovery request message, it sends a targetdiscovery response message, that is, a response to the target discoveryrequest message. Accordingly, each of the types of UE performs theoperation of the 2 step. In this aspect, the ProSe discovery process ofFIG. 14 may be called a 2-step discovery procedure.

In addition to the discovery procedure described in FIG. 14, if the UE 1(the discoverer) sends a discovery conform message (may be hereinafterabbreviated as an M3), that is, a response to the target discoveryresponse message, this may be called a 3-step discovery procedure.

In the present invention, a network node such as the terminal mayprovide a repeater function for another network node. The terminal thatprovides the relay function may be classified into a UE-NW repeater anda UE-UE repeater according to which network nodes the terminal providesthe relay function among.

FIG. 15 illustrates the UE-NW repeater.

Referring to FIG. 15, terminal 2 153 serves as the UE-NW repeater. Thatis, the terminal 2 53 is a network node which performs a relay betweenterminal 1 152 positioned outside coverage 154 of a network and thenetwork 151 and in this case, the terminal 2 153 may be referred to asthe UE-NE repeater.

In FIG. 15, since the terminal 1 152 is positioned outside the networkcoverage, when the terminal 2 153 does not provide the relay function,the terminal 152 may communication with the network 151.

The UE-NW repeater 153 transmits/receives data to/from the terminal 1through device-to-device communication (D2D operation) andtransmits/receives data through general terminal-to-networkcommunication.

FIG. 16 illustrates the UE-UE repeater.

Referring to FIG. 16, the terminal 2 163 serves as the UE-UE repeater.That is, the terminal 2 163 is a network node which performs a relaybetween terminal 1 152 positioned outside coverage 163 of a network andthe network 162 and in this case, the terminal 2 163 may be referred toas the UE-NE repeater.

In FIG. 16, since the terminal 1 162 and terminal 3 161 are positionedoutside the coverage, when the terminal 2 163 does not provide the relayfunction, the terminals 1 and 3 162 and 161 may communication with eachother. The UE-NW repeater 163 transmits/receives data to/from theterminal 1 through the device-to-device communication (D2D operation)and transmits/receives data to/from the terminal 3 through thedevice-to-device communication (D2D operation).

<Number of Hops of Network Node Providing Relay Function>

The network node providing the relay function is referred to as a relaynode. Then, the number of hops of the relay node may be defined as thenumber of communication links between the relay node and another networknode which becomes a target of the relay.

FIG. 17 illustrates the number of hops of the UE-NW repeater.

Referring to FIG. 17, the terminal 2 153 may be the network nodeproviding the relay function and another network node which becomes thetarget of the relay with respect to the terminal 1 152 may be a basestation 151. In this case, the number of hops of the terminal 2 153is 1. The reason is that the number of communication links between theterminal 2 153 and the base station 151 is 1. The network node providingthe relay function like the terminal 2 153 may signal the number ofhops. Therefore, the terminal 1 152 may find the number of hops of theterminal 2 153.

FIG. 18 illustrates the number of hops of another UE-NW repeater.

Referring to FIG. 18, the terminal 2 163 may be the network nodeproviding the relay function and another network node which becomes thetarget of the relay with respect to the terminal 1 162 may be a basestation 164. In this case, the number of hops of the terminal 2 163 is2. The reason is that the number of communication links between theterminal 2 163 and the terminal 3 161 is 1 and the number ofcommunication links between the terminal 3 161 and the base station 164is 1, and as a result, the total number of communication links betweenthe terminal 2 163 and the base station 164 is 2.

Hereinafter, the present invention will be described.

The present invention relates to which resource is used when the D2Doperation, for example, the D2D communication and the D2D discovery areperformed between the terminal outside the network coverage andterminals inside the network coverage.

FIG. 19 illustrates a case in which the present invention can beapplied.

Referring to FIG. 19, a cell which is the network may signal informationindicating resources represented by set B. Herein, it is assumed thatthe set B represents a resource pool which may be used for transmittinga D2D signal in the coverage (that is, the network coverage) of thecell. In addition, it is assumed that the terminal outside the networkcoverage may perform the D2D operation by using a predetermined resourcepool.

Terminal 1 (UE1) positioned outside the coverage of the cell moves toenter the coverage of the cell. In this case, when the terminal 1 andterminal 2 outside the coverage of the cell perform the D2D operation,which resource the terminal 1 will transmit the D2D signal by using maybecome an issue.

FIG. 20 illustrates a D2D operating method of a terminal according to anexemplary embodiment of the present invention.

Referring to FIG. 20, the terminal receives resource pool informationindicating a first resource pool which may be used for transmitting theD2D signal within the coverage of the cell from the cell (S210).

The terminal transmits the D2D signal by using a resource which overlapsbetween a second resource pool which may be used for the D2D operationoutside the coverage of the cell and the first resource pool (S220).

The terminal selects a specific resource within the resource whichoverlaps between the first resource pool and the second resource pool totransmit the D2D signal. For example, when the terminal transmits theD2D signal by mode 2, the terminal selects the specific resource withinthe resource which overlaps between the first resource pool and thesecond resource pool to transmit the D2D signal. The terminal maytransmit the D2D signal to another terminal outside the coverage of thecell by using the selected specific resource. Herein, the D2D signal maybe a signal for D2D communication or a signal for D2D discovery.

It is assumed that all terminals are, in advance, configured with theresource which may be used for the D2D operation outside the networkcoverage, that is, the second resource pool. A specific terminal mayenter the network coverage and receive the resource pool informationindicating the first resource pool which may be used for transmittingthe D2D signal within the network coverage. In this case, when thespecific terminal recognizes that the first resource pool and the secondresource pool overlap with each other, the specific terminal maytransmit the D2D signal by using some or all of the overlappedresources. Then, the resource used for the D2D transmission by thespecific terminal becomes a subset of the second resource pool.Therefore, other terminals positioned outside the network coverage mayreceive the D2D signal transmitted by the specific terminal withoutmodifying the second resource pool. In view of this, the methoddescribed in FIG. 20 is a method which may perform the D2D operationwithout transmitting the resource pool information to the terminaloutside the network coverage (cell coverage). Herein, the methoddescribed in FIG. 20 may be referred to as method 1.

FIG. 21 illustrates an example in which the method of FIG. 20, that is,the method 1 is applied.

Referring to FIG. 21, the cell which is the network may signal theinformation indicating the resources represented by the set B. Herein,the set B corresponds to the first resource pool of FIG. 20. Inaddition, it is assumed that the terminal UE 2 and 3 outside the networkcoverage may perform the D2D operation by using set A which is apredetermined resource pool. Then, the set A corresponds to the secondresource pool of FIG. 20.

The terminal 1 which enters the network coverage determines the partwhere the first resource pool and the second resource pool overlap witheach other and thereafter, when the overlapped part (an intersection) ispresent, a resource corresponding to the overlapped part is used as atransmission resource for transmitting the D2D signal. It may beregarded that the first resource pool is switched to the resourcecorresponding to the intersection of the sets A and B. Thereafter, theterminal 1 may transmit the D2D signal to the terminals 2 and 3 by usingthe resource corresponding to the intersection.

According to the method 1, the first resource pool used in the networkcoverage within the network coverage need not be announced to theterminal outside the network coverage. The reason is that a resourceactually used for transmitting the D2D signal within the networkcoverage is limited to the interconnection of the second resource pool.Therefore, even when the terminal 1 moves within the network coveragefrom the outside of the network coverage, the terminals 2 and 3 need notmodify the resource pools applied to the D2D operation and the D2Doperation does not stop.

However, according to the method 1, when the quantity of resourcesoverlapped between the first resource pool and the second resource poolis not sufficient, the quantity of resources which the terminal 1 withinthe network coverage may select for transmitting the D2D signaldecreases, and as a result, a degree of freedom of resource selection isreduced.

A method that transmits the D2D signal by using the resource overlappedbetween the first resource pool and the second resource pool may be usedfor facilitating communication between the terminal in the coverage ofthe cell and the terminal outside the coverage of the cell and used as aD2D transmission resource selection scheme for minimizing interferencewhich the terminal positioned at an area where coverages of differentcells using different resource pools overlap with each other applies.

FIG. 22 illustrates a D2D operating method of a terminal according toanother embodiment of the present invention.

Referring to FIG. 22, the terminal receives the resource poolinformation indicating the first resource pool which may be used fortransmitting the D2D signal within the coverage of the cell from thecell (S310).

When the resource is present, which overlaps between the second resourcepool which may be used for the D2D operation outside the coverage of thecell and the first resource pool, the terminal transmits the D2D signalby using the overlapped resource (S320).

When the resource is not present, which overlaps between the firstresource pool and the second resource pool, the terminal transfers theresource pool information indicating the first resource pool to anotherterminal outside the cell coverage (S330).

In the method described with reference to FIG. 22, in some cases, theresource pool information indicating the first resource pool may betransferred to the terminal outside the network coverage. Hereinafter,the method will be referred to as method 2.

FIG. 23 illustrates an example in which the method 2 is applied.

Referring to FIG. 23, similarly in the method 1, it is assumed that allterminals (UE 1, 2, and 3) are configured with the resource (that is,the second resource pool) which may be used outside the network coveragein advance. When the set B which may be used for transmitting the D2Dsignal within the network coverage does not overlap with the secondresource pool, the method 1 may not be applied and the method 2 may beapplied.

For example, when the terminal (UE1) enters the network coverage andrecognizes that the first resource pool and the second resource pool donot overlap with each other, the terminal 1 transfers the resource poolinformation indicating the first resource pool to the terminals (UE 2and 3) outside the network coverage. In addition, the terminal 1switches the resource for transmitting the D2D signal from the secondresource pool to the first resource pool (set B). When the terminals 2and 3 receive the resource pool information, the terminals 2 and 3switch the resource for the D2D operation from the second resource poolto the first resource pool (set B). When the terminal 1 transmits thefirst resource pool, the terminal may transmit information indicating aspecific time together. In this case, the terminal that receives thetime information starts using a first resource at an indicated time. Theterminal 1 also starts using the first resource according to theindicated time.

That is, when the external terminal outside the network coveragereceives resource pool information used by an internal terminal in thenetwork coverage, the external terminal may replace the resource pool(for the D2D operation) thereof with the resource pool indicated by theresource pool information. Then, the internal terminal and the externalterminal use a common resource pool for transmitting and receiving theD2D signal. Therefore, the internal terminal and the external terminalmay perform the D2D communication.

According to the method 2, the resource pool information indicating theresource pool used in the network coverage is transferred to theterminal outside the network coverage, and as a result, the internalterminal and the external terminal transmit and receive the D2D signalby using the same resource pool.

According to the method 2, when the terminal moves to the inside and theoutside of the network coverage, switching the resource pool isrequired. Therefore, when the terminal initially moves into and enterthe network coverage, the D2D communication may temporarily stop. Thereason is that a predetermined time will be required for decoding theresource pool information and switching the resource pool. However,since both the internal terminal and the external terminal use thecommon resource pool based on the network coverage, a higher degree offreedom than the method 1 is guaranteed when all terminals select theresource.

Meanwhile, in the method 2, when and how frequent the terminal transfersthe resource pool information in the cell coverage (network coverage),that is, the resource pool information indicating the first resourcepool to the terminals outside the cell coverage may become the issue.

FIG. 24 illustrates a terminal that transfers the resource poolinformation indicating the first resource pool to the terminals outsidethe cell coverage.

Referring to FIG. 24, when the terminal is positioned in a predeterminedrange or area within the cell coverage, whether the resource poolinformation within the coverage is transferred to the terminal outsidethe coverage may be defined.

For example, the terminal 1 may be positioned in a ring-shaped areadivided by two threshold values. The two threshold values may bemeasurement values for determining an inside boundary 251 and an outsideboundary 252 of the ring-shaped area. That is, the two threshold valuesmay include a first threshold value used for determining the insideboundary 251 of the cell coverage and a second threshold value used fordetermining the outside boundary 252 of the cell coverage. The thresholdvalues may be given as reference signal received power (RSRP) values,respectively and be signaled by the network or be predetermined.

Only when the terminal 1 is positioned between the inside boundary 251and the outside boundary 252, the resource pool information indicatingthe first resource pool may be transferred to the outside of the cellcoverage, that is, other terminals outside the outside boundary 252. Theterminal 1 may transfer the resource pool information by using resourcesmonitored by terminals outside the coverage. In order to increase aprobability that the terminals outside the coverage will receive theresource pool information, the terminal 1 may repeatedly transmit theresource pool information. The resource pool information may berepeatedly transmitted at a specific period and the specific period maybe configured by the network.

When the terminal 1 further moves to the center of the coverage than theinside boundary 251, a distance between the terminal 1 and the terminalsoutside the coverage increases, and as a result, transferring theresource pool information may be inefficient. Therefore, when theterminal 1 further moves to the center of the coverage than the insideboundary 251, the resource pool information is not transferred to theterminals outside the coverage.

A geographic positional range may be designated to the terminal indesignating a position where the terminal transfers the resource poolinformation in the coverage to the terminal outside the coverage. Inthis case, when a position of the terminal which the terminal measuresthrough a global positioning system (GP)/a global navigation satellitesystem (GNSS), and the like is included in the positional range, theterminal transmits the resource pool information.

The network may be configured to stop the terminal 1 to transfer theresource pool information to the terminals outside the coverage througha dedicated signal for the terminal 1. To this end, the terminal 1 maymake RRC connection with the network or make the RRC connection andthereafter, report the RRC connection to the network.

When a serving cell is not definitely defined or is to be changed, theterminal may consider not transferring the resource pool information inthe coverage to the terminals outside the coverage. For example, whenthe terminal is in a state other than a camped normally state, that is,an any cell selection state or a camped on any cell state, the terminaldoes not transfer the resource pool information in the coverage to theterminals outside the coverage. When the terminal may not camp on asuitable cell or an acceptable cell, the terminal does not transfer theresource pool information in the coverage to the terminals outside thecoverage. Alternatively, while the terminal performs a cell selectionoperation, the terminal does not transfer the resource pool informationin the coverage to the terminals outside the coverage.

When the terminal enters the network coverage, the terminal firsttransfers the resource pool information in the coverage to the terminalsoutside the coverage and after a predetermined time elapsed, theterminal may transmit the D2D signal by using the resource depending onthe transferred resource pool information. In this case, the terminalmay repeatedly transfer the resource pool information in the coverage inorder to definitely successfully transfer the resource pool informationin the coverage to other terminals outside the coverage.

Alternatively, when the terminal enters the network coverage, theterminal may use the resource depending on the resource pool informationin the coverage before transferring the resource pool information in thecoverage to the terminals outside the coverage. Thereafter, the terminalmay transfer the resource pool information in the coverage to theterminals outside the coverage. In this case, the resource poolinformation in the coverage may be provided through the resourcemonitored by the terminals outside the coverage, that is, the secondresource pool.

FIG. 25 illustrates a D2D operation of a terminal according to yetanother embodiment of the present invention.

Referring to FIG. 25, the network (cell) transmits the resource poolinformation indicating the first resource pool and scheme information tothe terminal 1 (S401). The network may configure which method of themethods 1 and 2 is used with respect to the terminal 1 through thescheme information. The network may, for example, signal the schemeinformation indicating which method of the methods 1 and 2 is used,which is included in system information. When the terminal 1 moves intoand enters the network coverage, the terminal 1 may find a scheme to beapplied by receiving the scheme information included in the systeminformation.

The terminal 1 selects a scheme depending on the scheme information(S402). That is, either one of the method 1 described with reference toFIG. 20 or the method 2 described with reference to FIG. 22 may beselected.

The terminal 1 transmits the D2D signal by using a resource depending onthe selected scheme to the terminal 2 outside the cell coverage (S403).

Meanwhile, as illustrated in FIG. 25, the network may provide athreshold value and allow the terminal itself to select any method ofthe methods 1 and 2 based on the threshold value instead of directlyconfiguring which method of the methods 1 and 2 is used. For example,the network may provide the threshold value for a part where apredetermined second resource pool and the first resource pool used inthe coverage overlap with each other. The terminal determines the partwhere the second resource pool and the first resource pool overlap witheach other and thereafter, compare the threshold values. When thequantity of resources which overlap between the first resource pool andthe second resource pool is smaller than the threshold value, theterminal transfers the resource pool information indicating the firstresource pool to another terminal outside the cell coverage. That is,the method 2 is used. On the contrary, when the overlapped part islarger than the threshold value, the method 1 may be applied.

Meanwhile, it is assumed that the terminal that performs the D2Doperation by a specific method outside the network coverage moves intoand enters the network coverage of a specific cell and thereafter, campson the specific cell.

In this case, the terminal may examine whether a specific indication ispresent in the system information. The specific indication may indicatethe specific scheme operation of the terminal to be allowed until thenetwork explicitly does not allow the specific scheme outside thenetwork coverage. Herein, the specific scheme may indicate a scheme (forexample, by mode 1 or 2) that selects the resource for the D2Doperation. For example, when the terminal which performs the D2Doperation in mode 2 outside the network coverage enters the networkcoverage of a specific cell and camps on the specific cell, it may beregarded that the operation of mode 2 is allowed until the specific celldoes not explicitly allow the operation of mode 2 through the specificindication. Alternatively, the specific indication may indicate themode-2 operation of the terminal to be allowed until the networkexplicitly configures the operation of mode 1. Alternatively, thespecific indication may indicate the operation of mode 2 to be allowedunless the network configures the D2D communication through thededicated signal for the terminal.

When the specific indication is present, the terminal continuouslyoperates by the specific method outside the network coverage. When thespecific indication is not present, the terminal stops the operation bythe specific method outside the network coverage.

Alternatively, when the resource pool information which may be appliedto the specific method outside the network coverage is signaled by thecell, the terminal may regard that the specific method outside thenetwork coverage is allowed even within the network coverage. When thenetwork does not explicitly allow the specific method outside thenetwork coverage, the network explicitly configures a different methodfro the specific method outside the network coverage, or the networkdoes not configure the D2D communication through the dedicated signalfor the terminal, the terminal may regard that the specific method isallowed outside the network coverage.

Alternatively, the terminal may stop the specific method outside thenetwork coverage in the network coverage regardless of whether theresource pool information which may be applied to the specific methodoutside the network coverage is signaled by the cell.

FIG. 26 is a block diagram illustrating a terminal in which anembodiment of the present invention is implemented.

Referring to FIG. 26, a terminal 1100 includes a processor 1100, amemory 1120, and a radio frequency (RF) unit 1130. The processor 1110implements a function, a process, and/or a method which are proposed.For example, the processor 1110 receives from a cell resource poolinformation indicating a first resource pool which may be used fortransmitting a D2D signal within coverage of a cell and transmits theD2D signal by using a resource which overlaps between a second resourcepool which may be used for a D2D operation outside the coverage of thecell and the first resource pool.

The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for a device-to-device (D2D) operationperformed by a terminal in a wireless communication system, the methodcomprising: receiving resource pool information indicating a firstresource pool usable for transmitting a D2D signal within coverage of acell from the cell; and transmitting the D2D signal by using a resourcewhich overlaps between a second resource pool usable for the D2Doperation outside the coverage of the cell and the first resource pool.2. The method of claim 1, wherein the terminal selects a specificresource within the resource which overlaps between the first resourcepool and the second resource pool to transmit the D2D signal.
 3. Themethod of claim 2, wherein the terminal transmits the D2D signal toanother terminal outside the coverage of the cell by using the selectedspecific resource.
 4. The method of claim 3, wherein the D2D signal maybe a signal for D2D communication or a signal for D2D discovery.
 5. Themethod of claim 1, wherein when the resource is not present, whichoverlaps between the first resource pool and the second resource pool,the terminal transfers the resource pool information indicating thefirst resource pool to another terminal outside the cell coverage. 6.The method of claim 1, wherein scheme information indicating any onescheme of a first scheme and a second scheme is received from the cell,and whether the resource pool information is transferred to anotherterminal outside the cell coverage is determined according to the schemeindicated by the scheme information.
 7. The method of claim 6, whereinthe first scheme is a scheme in which the terminal transmits the D2Dsignal by using the resource which overlaps between the first resourcepool and the second resource pool without transferring the resource poolinformation to another terminal outside the cell coverage, and thesecond scheme is a scheme in which the terminal transmits the D2D signalby using the first resource pool by transferring the resource poolinformation to another terminal outside the cell coverage.
 8. The methodof claim 6, wherein the scheme information is included in systeminformation.
 9. The method of claim 1, wherein a threshold value for theoverlapped resource is received from the cell.
 10. The method of claim9, wherein when the quantity of resources which overlap between thefirst resource pool and the second resource pool is smaller than thethreshold value, the resource pool information indicating the firstresource pool is transferred to another terminal outside the cellcoverage.
 11. The method of claim 1, wherein the threshold value for thecell coverage is received from the cell.
 12. The method of claim 11,wherein the threshold value for the cell coverage includes a firstthreshold value used for determining an inside boundary of the cellcoverage and a second threshold value used for determining an outsideboundary of the cell coverage.
 13. The method of claim 12, wherein onlywhen the terminal is positioned between the inside boundary and theoutside boundary, the resource pool information is transferred toanother terminal outside the cell coverage.
 14. The method of claim 13,wherein the terminal periodically transfers the resource poolinformation.
 15. A terminal comprising: a radio frequency (RF) unittransmitting and receiving a radio signal; and a processor operated inassociation with the RF unit, wherein the processor receives resourcepool information indicating a first resource pool usable fortransmitting a D2D signal within coverage of a cell from the cell, andtransmits a D2D signal by using a resource which overlaps between asecond resource pool usable for a D2D operation outside the coverage ofthe cell and the first resource pool.